1. Field of the Invention
The present invention relates to an apparatus for measuring battery voltage. In particular the invention relates to an apparatus for measuring battery voltage that, where a battery cell is composed of a plurality of secondary batteries connected in series and a plurality of these battery cells are divided into groups of an appropriate number for modularization, detects the battery voltage of each module accurately.
This application is based on Japanese Patent Application No. Hei 11-300205 (Unpublished), the contents of which are incorporated herein by reference.
2. Description of the Related Art
Heretofore there is known a hybrid vehicle providing with a motor in addition to an engine as a drive source for vehicle running.
One type of such hybrid vehicle is a parallel hybrid vehicle where the motor assists the output from the engine. With this parallel hybrid vehicle, for example at the time of acceleration, the output from the engine is assisted by means of the motor, while at the time of deceleration, various control is carried out such as performing charging of a battery by deceleration regeneration, so that the remaining battery charge (state of charge) can be maintained while satisfying the requirements of the driver. Here, this battery, since high voltage is required, is normally composed of a plurality of battery cells connected in series.
Heretofore when detecting the voltage of this battery, a plurality of battery cells connected in series comprising the battery is divided into groups of an appropriate number for modularization, the voltage of each module is detected, and the sum of the detected voltages constitutes the voltage of the battery.
For the detecting circuit to detect this voltage, for example, a circuit in which a differential amplifier and an insulated type DC/DC converter are combined has been used (Japanese Unexamined Patent Application, First Publication No. Hei 11-113182)
However, with an apparatus for measuring battery voltage that is realized by using a differential amplifier and an insulated type DC/DC converter, the gain is limited by the common mode input voltage range of the amplifier used, and any error in the resistance used has a major influence on the accuracy of the whole amplifier. Therefore there is a drawback in that high accuracy is difficult.
Furthermore, when constructing this, the amplifier used for the differential amplifier is very expensive, and in addition a plurality of insulated type DC/DC converters is needed as a power source. Therefore there are disadvantages of high cost and also difficulty of miniaturization.
Moreover, in the case where a voltage measuring line of the battery is continuously connected to the battery voltage measuring circuit, since dark current of the circuit is associated with discharge of the electric charge of the battery, the circuit must be designed such that dark current is minimized. Therefore there are many design limitations in realizing a high accuracy apparatus for measuring voltage.
The present invention takes into consideration such conditions, with the object of providing an apparatus for measuring battery voltage that can detect battery voltage with high accuracy, and also can be miniaturized with cost and noise reduced. Furthermore, another object of the present invention is to provide an apparatus for measuring battery voltage that has lower dark current even when continuously connected to the secondary battery, and can thus minimize the voltage reduction of the secondary battery even after the secondary battery is left unused for a long time.
To achieve the aforementioned objects, the present invention is an apparatus for measuring battery voltage (voltage detecting circuit 20 in an embodiment), which divides a plurality of batteries (secondary battery 30) connected in series into a plurality of blocks (modules 10-1, 10-2, 10-N) comprising at least one battery, and measures the voltage of the blocks. It is characterized in that there is provided: a power storage element (capacitor 101) installed in parallel for each of the blocks; a voltage acquisition circuit (voltage follower 301) for obtaining an output voltage corresponding to the voltage across the power storage element; a first switching element group (switches 201 and 202) comprising a plurality of switching elements, which is inserted between the power storage element and the block for connecting the power storage element and the block in parallel; a second switching element group (switches 203 and 204) comprising a plurality of switching elements, which is inserted between the power storage element and the voltage acquisition circuit for connecting the power storage element and the voltage acquisition circuit; and a driver (CPU (central processing unit) 700) for switching the first switching element group and the second switching element group alternately.
With this construction, when detecting the voltage of a block, no expensive element needs to be used. Therefore it is possible to realize an apparatus for measuring battery voltage that can be miniaturized at low cost. Furthermore, after the voltage of the block has charged the capacitor, it is input via a voltage acquisition circuit (voltage follower 301 in the embodiment) to an A/D (analogue to digital) converter installed thereafter, thus enabling high accuracy voltage detection.
In addition, when a capacitor is charged to the same electric potential as the corresponding block, almost no current flows. Therefore the dark current can be reduced even when a voltage measuring line of the battery is continuously connected to the battery voltage measuring circuit. In this manner it is possible to minimize reduction of the charge of the battery even after the battery is left unused for a long time.
Furthermore, the driver of the present invention, at a time of switching the first switching element group and the second switching element group, after turning a switching element group which has been on off, turns the other switching element group on, and also when the first switching element group is on, after the voltage of the power storage element has reached the same voltage as the block, turns the first switching element group off.
By driving the switching elements in this manner, that is to say, by setting a period when all switching elements are off, it is possible to avoid a situation where all switching elements are on at the same time due to a factor such as variation in the speed of response. As a result, it is possible to detect the voltage of the block accurately according to a predetermined timing. Here, it is necessary to set a period when all switching elements are off that is longer than the speed of response of the switching elements used.
Furthermore, an input impedance of the voltage acquisition circuit is infinite. With this construction, with simple and miniature elements it is possible to detect a voltage with the least possible discharge of electric charge from the capacitor. In this manner, it is possible to measure the voltage to be measured more accurately. As a result, a low cost, miniaturized and high accuracy apparatus for measuring battery voltage can be realized.
Additionally, the switching elements are bidirectional and insulated from a power source for driving the switching elements,
With the apparatus for measuring battery voltage of the present invention, the voltage across the block, after charging the capacitor, is input via the voltage follower to the A/D converter etc. installed thereafter. Here, since the input impedance of the voltage acquisition circuit is infinite, the electric charge stored in the capacitor is not discharged. Consequently, until the capacitor and the block are connected again, the electric charge corresponding to the previous voltage of the block is stored by the capacitor. Therefore, depending on the electric potential of the block and the electric charge that the capacitor holds, the capacitor may be charged or discharged. As a result, for a switching element to connect the block and the capacitor, a bidirectional switching element is necessary.
Moreover, when detecting the voltage of a high voltage system power supply unit, since high voltage is to be measured, voltage leakage thereby must be avoided. Therefore by using switching elements, whereby the power supply system to drive the switching elements and the circuit system to which the switching elements are connected are insulated, it is possible to insulate the circuit system and the drive system of the switching elements easily.
Furthermore, the switching elements are constituted by photo MOSFETs (Metal Oxide Semiconductor Field Effect Transistors). Here, photo MOSFETs are components that are electrically insulated from the power supply system that drives the photo MOSFETs. Accordingly, it is possible to insulate the power supply system that drives the switching elements and the high voltage system such as a battery. For example, heretofore in the case when applied to an electric vehicle or the like, an insulated type DC/DC converter etc. has been used to insulate the 12V battery system, being a power source to drive the photo MOSFETs, and the battery system for the high voltage system comprised of batteries to be measured. Here, by using a photo MOSFET instead of this DC/DC converter, it is possible to insulate the power source system and the battery voltage system with an inexpensive and miniaturized element. Therefore, the circuit construction can be simplified, so that it is also possible to achieve cost reduction and miniaturization of the circuit.
Furthermore, in the case where the battery voltage is lower than the electric potential of the capacitor, discharge is performed from the capacitor to the battery, so that a bi-directional switch such as a photo MOSFET is suitable. Moreover, since the voltage drop of this photo MOSFET at the time of current flow is small, it is possible to output the battery voltage to the voltage detecting circuit accurately.
Furthermore, the voltage acquisition circuit of the present invention may be comprised of a voltage follower. Therefore, with simple and miniaturized elements, it is possible to remove the noise factor of the voltage to be measured from the capacitor. As a result, a low cost, miniaturized and high accuracy apparatus for measuring battery voltage can be realized. Additionally, since the voltage of the block is input via the voltage follower (a constant voltage circuit) to the circuit such as an A/D converter installed thereafter, an accurate voltage is detected as the voltage of the block with noise removed.